Receiving information in the presence of interference has long been a problem in the field of communications. In particular, operation of a receiver in a jamming environment can be especially difficult since a variety of jamming techniques are available which the receiver must contend with. In response to this, an impressive array of antijam (AJ) techniques have been developed. Certainly, one of the most successful developments in this regard has been the introduction of spread spectrum systems which spread the desired transmitted information over a very wide bandwidth by modulating a carrier with a wideband encoding signal. The actual encoding scheme can be selected from one of a variety of options (e.g., chirp, QPSK, FFH, etc.) to permit reception of information in an environment of heavy jamming. The receiver, of course, must be suited to despread and decode the information utilizing circuitry appropriate for the particular encoding scheme employed by the transmitter. A major advantage of such spread spectrum systems is that they permit receiving desired information in situations where a jamming signal is considerably greater than the information signal itself.
Notwithstanding the marked advantages of spread spectrum systems, there is still room for improvement of spread spectrum receivers since, often, certain types of jamming can be quite successful in obscuring information sent with a spread spectrum system. In particular, in some cases, the received information has a power level so far below the jamming level that it becomes quite difficult to fully decipher the desired information. One type of jamming which is often relatively effective is constant envelope jamming which utilizes a signal where the instantaneous power remains substantially fixed. The envelope of such a constant envelope jammer will have no substantial variation, although the phase of the carrier can change. Other techniques which can be difficult to receive information through include systems where plural sinewaves are superimposed on one another to provide a varying envelope or systems which modulate a carrier to produce a uniformly distributed envelope where all envelope levels are equally likely (e.g., as in the case of triangular modulation). These and other jamming techniques can produce either substantially constant envelopes (if the variation is slow relative to the time required for the receiver to receive a single data symbol) or a rapidly varying envelope. In any event, although spread spectrum techniques can be successful against such schemes, these jamming techniques still create considerable difficulties in receiving the information.